This invention relates to high intensity arc discharge lamps and more particularly to high intensity arc discharge metal halide lamps having high efficacy.
Due to the ever-increasing need for energy conserving lighting systems that are used for interior and exterior lighting, lamps with increasing lamp efficacy are being developed for general lighting applications. Thus, for instance, arc discharge metal halide lamps are being more and more widely used for interior and exterior lighting. Such lamps are well known and include a light-transmissive arc discharge chamber sealed about an enclosed a pair of spaced apart electrodes, and typically further contain suitable active materials such as an inert starting gas and one or more ionizable metals or metal halides in specified molar ratios, or both. They can be relatively low power lamps operated in standard alternating current light sockets at the usual 120 Volts rms potential with a ballast circuit, either magnetic or electronic, to provide a starting voltage and current limiting during subsequent operation.
These lamps typically have a ceramic material arc discharge chamber that usually contains quantities of metal halides such as CeI3 and NaI, (or PrI3 and NaI) and T1I, as well as mercury to provide an adequate voltage drop or loading between the electrodes and the inert starting gas. Such lamps can have an efficacy as high as 105 LPW at 250 W with a Color Rendering Index (CRI) higher than 60, with Correlated Color Temperature (CCT) between 3000 K and 6000 K at 250 W.
Of course, to further save electric energy in lighting by using more efficient lamps, high intensity arc discharge metal halide lamps with even higher lamp efficacies are needed. The lamp efficacy is affected by the shape of the arc discharge chamber. If the ratio between the distance separating the electrodes in the chamber to the diameter of the chamber is too small such as being less than two, the relative abundance of Na between the arc and the chamber walls leads to a lot of absorption of generated light radiation by such Na due to its absorption lines near the peak values of visible light. On the other hand, if the ratio between the distance separating the electrodes in the chamber to the diameter of the chamber is too great such as being greater than five, initiating an arc discharge in the arc discharge chamber is difficult because of the relatively large breakdown distance between the electrodes. In addition, such lamps perform relatively poorly when oriented vertically during operation in exhibiting severe colors segregation as the different buoyancies of the lamp content constituents cause them to segregate themselves from one another to a considerable degree along the arc length.
Another shape consideration is the avoidance of discontinuities in the chamber inner surface such as the presence of corners in the vicinity of the meeting locations of the chamber ends and the chamber central portion, or overlapping joint walls therebetween of similar thicknesses, which discontinuities, if present, result in “cold spots” in the chamber plasma during lamp operation which lowers vapor pressures in the chamber to thereby reduce radiant flux therefrom. In addition, the chamber ends must be shaped so as to leave sufficient clearance between the walls thereof and the electrodes so that temperatures of the ends does not get so great as to damage the structural integrity of those walls. Thus, there is a desire for an arc discharge chamber that strongly emits light radiation of good color while being operable by currently used ballast circuits.